The present invention relates to a seat belt tensioning device to be installed on a vehicle.
Conventionally, the seat belt tensioning device of this type is provided with a cylinder, a piston which is slidably inserted within the cylinder and is directly or indirectly connected to a seat belt and a piston operating means which moves the piston within the cylinder to tension the belt at an urgent or critical time such as upon a vehicle impact. When the piston is moved to tension the seat belt by the piston operating means upon a vehicle impact, the seat belt is tensioned to hold the passenger on his seat. As the piston operating means, gas pressure or the like is utilized.
In one example of the conventional seat belt tensioning device of this type, the seat belt is connected to the piston through a deformable member which is plastically deformed under a predetermined load. Upon a vehicle impact, the seat belt is elongated by the deformable member only to such an extent that the passenger does not collide against the front panel of the vehicle and the like, i.e., a secondary impact. The kinetic energy of the passenger is absorbed in the elongating process of the seat belt.
However, in the above described conventional seat belt tensioning device, it is very difficult to adjust the tension force of the seat belt to a precise desired value by the piston operating means utilizing gas pressure or the like. In other words, a constant tension force cannot always be obtained.
If the tension force of the seat belt is small, the passenger is in danger of a secondary impact. If the tension force becomes too large, the passenger is in danger of being injured by the tensioned seat belt itself.
In the conventional seat belt tensioning device of the type wherein the kinetic energy of the passenger is absorbed by the deformable member, the piston operating force produced by the piston operating means is not always maintained constant. Therefore, if the piston operating force is too large, the deformable member starts to be elongated while the piston is being moved by the piston operating means. As a result, the desired extension of the deformable member for absorbing the kinetic energy of the passenger cannot be obtained.
Accordingly, one object of the present invention is to provide a seat belt tensioning device wherein the tension load of the seat belt applied to the passenger can be precisely adjusted to a desired value regardless of variations of the piston operating force of the piston operating means.
Another object of the present invention is to provide a seat belt tensioning device in which an energy absorbing member effectively operates to absorb the kinetic energy of the passenger.